It is known to arrange heave compensators in the interface between a floating installation and a riser extending from the sea floor up to the installation. The heave compensator keeps the riser in the correct vertical position in the water while letting the floating installation move vertically with respect to the riser due to waves, swells, and tide. This is typically the case on a drilling riser (low pressure), while on an intervention riser (high pressure) such riser is typically running up to a flow tree in derrick. From the perspective of the personnel on the heaving floating installation, such intervention riser is moving up and down. Performing manual work on the top of the riser is therefore undesirably hazardous, as large movements and large forces are active. To do such work, it is known to suspend personnel to structures that are not moving with respect to the riser, so-called man-riding. This is only permitted in rather calm sea, thus limiting the time scopes for when such operations can be performed.
To avoid such working conditions, it is known to install a slip joint above the tension joint of the riser. With the slip joint in a telescoping mode, the upper part of the riser is fixed to the floating installation, preventing vertical movement between the installation and the upper part of the riser. Manual work can then be performed more safely.
Several publications describe such slip joints for use with intervention risers, one being international patent application publication WO 03067023 (Blakseth). This publication describes an arrangement for well completion and intervention operations where a workover riser (4) is projecting from a wellhead (6) and up to a vessel (2), and where the upper portion of the workover riser (4) is designed to be displaced from an upper position to a lowered position for rigging work. In this lowered position the upper displaceable portion of the riser (4) essentially follows the heave movements of the vessel. After the rigging work, the displaceable portion of the riser is again raised to the upper position, the riser being equipped with a telescoping connection (1). Before lowering the upper portion of the riser to the working position, pressure is relieved.
Publication WO 0024998 (Baker Hughes Incorporated) describes a pressurized slip joint for a marine intervention riser that decouples a flow head assembly in the moon pool of a vessel from the riser string to enable safe changeover of equipment during workover operations. One part of the slip joint assembly is coupled to the flow head assembly through a flexible joint assembly. A second part of the slip joint assembly supports the riser string and is coupled to the tensioning mechanism. The first part may be inserted into the second part and locked in place during workover operations, except when equipment changeover is taking place. The first and second parts have an unlocked and a locked mode. When in the unlocked mode, a low pressure seal is used, whereas a high pressure seal is used in the locked mode. When the first part is inserted into the second part, in the locked mode, a high pressure metal seal seals between the lower part of the first part and a shoulder inside the second part. Thus, the first part retains high pressure in this mode.
Common for the solutions described in these publications is that they consist of exactly the same building elements as a conventional telescopic joint used in all drilling risers. This is: an outer sleeve, an inner sleeve, inner sleeve running inside outer sleeve, a latch between inner and outer sleeve and seal arrangements between inner and outer sleeve. The main difference between above mentioned publications and such prior art drilling riser telescopic joint is that they are designed to withstand high pressure, not only low pressure as on a drilling riser telescopic joint.
The main functional difference between the two referenced publications is that WO 03067023 is in fully stroked out position when pressurized with high pressure and WO 0024998 is in fully retracted position when pressurized with high pressure.
Common for both WO 03067023 and WO 0024998 is further that both the inner and outer sleeves retains high pressure fluid when not telescoping (in the locked mode), whereas the pressure is relieved when in the telescoping mode. Thus, the inner telescoping pipe must be dimensioned to withstand such high pressure even though such pressure is not present when the inner pipe is fulfilling its main purpose, namely the telescoping action. Furthermore, the outer sleeve needs to be of a large dimension in order to accommodate the size of a high pressure inner sleeve. Thus, superfluous material is used resulting in increased weight and costs. And not at least, a very stiff riser is very disadvantageous with respect to the resulting high bending moment of the riser through rotary/work floor/moonpool and hence a stiff riser offers a very limited fatigue life of the riser. Both WO 03067023 and WO 0024998 includes both inner and outer barrels that will have to withstand either/or the full riser tension and internal high pressure.
The arrangement described in WO 03067023 exhibits still a further disadvantage, since the upper telescoping part must be in an upper position when in the non-telescoping or locked mode. This makes the upper part extend disadvantageously far up, making the necessary connections and connected devices, such as a surface flow tree, being arranged inconveniently high with respect to the floating installation. The slip joint will also be in extended mode when installing it, thereby requiring a large lifting height of the derrick.